1. Field of the Invention
The present invention relates to a digital-to-analog converter, and more particularly, to a non-linear digital-to-analog converter which realizes non-linear output with overlapped resistor string.
2. Discussion of the Related Art
Generally, industrial systems and controllers have trends to digitalization with the spread of CPUs having fast speed in an analog circuit at suitable cost. Most of industrial systems and controllers share digital mode with analog mode. In this respect, for signal transmission between digital mode and analog mode, an analog-to-digital converter or a digital-to-analog converter is required.
An analog-to-digital converter is used in the field of instrumentation and control. With the development of microcomputers, parts processed by analog mode have been processed by converting an analog signal to a digital signal using the analog-to-digital converter. For this reason, reliability and efficiency of applied products have improved.
A background art digital-to-analog converter will be described with reference to the accompanying drawings.
FIGS. 1a and 1b are schematic diagrams illustrating a background art digital-to-analog converter.
As shown in FIG. 1a, the background art digital-to-analog converter includes a primary resistor string 1, most significant bit (MSB) switching block 2, least significant bit (LSB) switching block 3, a secondary resistor string 4, and LSB output switching portion 5.
Two voltages formed in succession between V2 and V1 are applied between M1.about.M7 of the primary resistor string 1 using the MSB switching block 2 and the LSB switching block 3. One voltage is output through the LSB output switching portion 5.
As shown in FIG. 1b, the background art digital-to-analog converter includes a primary resistor string 6 consisting of Ra1.about.Ra2.sup.N/2 -1, a first switching block 7 consisting of Sa1-Sa2.sup.N/2 1, a secondary resistor string 8 consisting of Rb1.about.Rb2.sub.N/2 -1, a second switching block 9 consisting of Sb1.about.Sb2.sup.N/2 -1, a cubic resistor string 10 consisting of Rc1.about.Rc2.sup.N/2 -1, and a third switching block 11 consisting of Sc1.about.Sc2.sup.N/2 -1.
In the aforementioned digital-to-analog converter, resistor ratio of Ra, Rb and Rc is 2.sup.N/2 :2.sup.N/2 :1 and a total of resistor string selected by the MSB plus resistor string selected by the LSB is 2.sup.the number of MSB *2.sup.N/2 of Rc.
In other words, among 6 bits, when high 3 bits are the MSB and low 3 bits are the LSB, Ra is 2*2.sup.3 times (8-101.sub.(2) -1) of Rc in case of 101010.sub.(2). At this time, Rb is 5*2.sup.3 times (101.sub.(2)). Since Rc is always 2.sup.3, total resistor value is 8*2.sup.3 times (2.sup.3 *2.sup.3).
If the value of 010.sub.(2) is selected by Sc of the third switching block 11, it is possible to wholly select the value of 101010.sub.(2).
The aforementioned background art digital-to-analog converter has several problems.
First, since the secondary resistor string is always fixed at a uniform value, it can be applied to the digital-to-analog converter which outputs linear value. However, it cannot be used in apparatuses which require non-linear value. In addition, the resistor value may be greater than a desired resistor value due to on-resistor constituting each switching block. This causes incorrect conversion value.